from an Increase of Temperature. 359 



openings of these stop-cocks, the thermometers may easily be 

 kept at the same temperature for any length of time. MM are 

 the micrometer microscope, strongly fixed to the stand A, and 

 opposite to plate-glass windows W, Fig. 2, which are placed in 

 the double case B, so as to suit the lengths of the rods of which 

 it was wished to measure the expansion. The upper micrometer 

 reads the thirty-thousandth part of an inch, and the under one 

 was intended to have been used to read a much smaller quanti- 

 ty ; but it required great care to draw the lines on the silver 

 studs on which the lengths were laid off, so as to make the up- 

 per micrometer read to the same division ; it was therefore found 

 more convenient to employ the under micrometer as a fixed one, 

 by moving the zero line of the under stud on the rod, till it in- 

 tersected the angle of the cross spider-threads in the under mi- 

 crometer. This was done by the screw G working on the long 

 end of a lever, which raised and lowered the rod under experi- 

 ment ; the lever, besides reducing the quickness of the screw G, 

 gave it a more convenient position, and made the rod move 

 without any tendency to turn. The whole body of the double 

 case B was covered with a jacket, about the third of an inch 

 thick, made of several plies of flannel ; and all the metal parts 

 of the top and bottom, as well as the upper stop-cock E, and 

 the greater part of the upper thermometer, were also thickly 

 covered with flannel. This was found necessary to get the up- 

 per and under thermometer to stand at the same tempera- 

 ture. There was a screen R of polished tin-plate interposed be- 

 tween the stand supporting the micrometer microscopes and the 

 double case B, in order to prevent any heat from being radiated 

 from the one to the other. Also, in order to keep the stand of 

 the pyrometer at the same temperature during the experiment, 

 it was placed near the window of the room, and the furnace for 

 the boiler was built in a common fire-place, the front of which 

 was closed with a screen of thick boards ; and the polished tin- 

 pipes which conducted the steam from the boiler to the instru- 



